Field of the Invention
Embodiments of the invention generally relate to the fabrication of solar, semiconductor, and electronic devices, and more particularly to epitaxial lift off (ELO) devices and methods.
Description of the Related Art
One phase in device fabrication involves handling and packaging of thin films used as solar devices, semiconductor devices, or other electronic devices. Such thin film devices may be manufactured by utilizing a variety of processes for depositing and removing materials onto a wafer or other substrate. One uncommon technique for manufacturing thin film devices is known as the epitaxial lift off (ELO) process. The ELO process includes depositing an epitaxial layer or film on a sacrificial layer on a growth substrate, then etching the sacrificial layer to separate the epitaxial layer from the growth substrate. The thin epitaxial layer removed is known as the ELO film or layer and typically includes thin films used as solar devices, semiconductor devices, or other electronic devices.
The thin ELO films are very difficult to manage or handle, such as when bonding to a substrate or while packaging, since the ELO films are very fragile and have narrow dimensions. The ELO films crack under very small forces. Also, the ELO films are very difficult to align due to their extremely narrow dimensions.
The sacrificial layer is typically very thin and is usually etched away via a wet chemical process. The speed of the overall process may be limited by the lack of delivery or exposure of reactant to the etch front, which leads to less removal of by products from the etch front. This described process is a diffusion limited process and if the films were maintained in their as deposited geometries, a very narrow and long opening would form to severely limit the overall speed of the process. To lessen the transport constraint of the diffusion processes, it may be beneficial to open up the resulting gap created by the etched or removed sacrificial layer and bending the epitaxial layer away from the growth substrate. A crevice is formed between the epitaxial layer and the growth substrate—which geometry provides greater transport of species both towards and away the etch front. Reactants move towards the etch front while by-products generally move away from the etch front.
The bending of the epitaxial layer however can induce stresses there within and the amount of bending is limited by the strength of the film. The epitaxial layer usually contains a brittle material, which does not undergo plastic deformation before failure, and as such may be subject to crack induced failures.
To minimize the potential for crack propagation, the brittle epitaxial layer may be maintained under a compressive stress. Cracks usually do not propagate through regions of residual compressive stress. The epitaxial layer is placed under tensile stress while bending the epitaxial layer away from the growth substrate since the epitaxial layer is on the outside of the curvature of the crevice. The tensile stress limits the amount of crevice curvature and reduces the speed of the etch process. To overcome this limitation, a residual compressive stress may be instilled within the epitaxial layer before etching the sacrificial layer. This initial compressive stress may be offset by tensile stress caused by the bending and therefore allows for a greater amount bending during the separation process.
Therefore, there is a need for more robust ELO thin films, as well as for methods to form, remove, and handle ELO thin films.